1. Field of the Invention
This invention relates to optical fiber claddings and more particularly to optical fibers comprising extrudable or solution coatable low refractive index optical fiber cladding.
2. Description of the Related Art
Optical fibers have received widespread interest for information and data transfer. Fiber-guided modulated light beams are useful in many applications, for example, telecommunications, computer link-ups, and automotive controls. Advantageously, fiber optic linkages have a greater information carrying capacity as compared to metal wires carrying electrical signals. Furthermore, fiber optics are less likely to suffer from external interference, such as electromagnetic radiation.
Typically, optical fibers comprise a light carrying core, for example an inorganic glass such as fused silica or a polymer such as polymethyl methacrylate, and a cladding material having a lower refractive index than the core. The cladding material serves to confine the light energy within the core and thereby allows propagation of light by a phenomenon generally known as "total internal reflection."
Characteristically, glass optical fiber cores have very low optical loss and are generally preferred for long distance applications. On the other hand, the cost of connecting glass optical fiber cores tend to be cost prohibitive for short distance, connector intensive applications. Polymer fibers overcome the cost limitation for short distances. Furthermore, they are lighter weight, more flexible, and have a larger diameter than glass fibers. Although polymer fibers exhibit a greater optical loss than glass core fibers, they are preferred in shorter length applications. The most common commercial polymer optical fiber core material is polymethyl methacrylate (PMMA).
Transmission in optical fibers is typically improved by the use of a "cladding" having a lower refractive index to promote total internal reflection. Thus, the cladding acts like a mirror to keep the light in the fiber and reduces scattering losses of the optical fiber.
EPO 250,996 describes .alpha.- and .beta.-fluorinated acrylates and methacrylates as cladding materials for glass core optical fibers.
EPO 256,765 describes .alpha.- and .beta.-fluorinated acrylates and methacrylates as a cladding composition for polymer core optical fibers wherein the cladding composition comprises (a) ultraviolet ray-cured monofunctional acrylic or .alpha.-fluoroacrylic monomer, (b) a polyfunctional acrylate or .alpha.-fluoroacrylate having at least two acryloyl or .alpha.-fluoroacryloyl radicals in the molecule and (c) a photoinitiator. The cladding composition is first flow coated onto an optical fiber core and then ultraviolet ray-cured.
W. Groh, "Overtone Absorption in Macromolecules for Polymer Optical Fibers", Makromol. Chem., 189, 2861, 1988, a paper describing molecular bond absorption, concludes that perfluorinated polymers should show low optical loss.
Kokai No. JP60-258281 (English translation) describes optical lenses consisting of copolymers of at least one polymerizable compound containing polyfluoroalkyl methacrylates and non-fluorinated methacrylates.
Japanese Patent No. 62-208005 (English translation) describes optical fibers formed from polymers expressed by the general formula ##STR1## wherein R.sup.1 is a hydrogen atom, or C.sub.1-3 alkyl groups, X is a halogen atom and m is an integer of 1-5.
U.S. Pat. No. 4,500,694 (Ohmori et al.) describes optical fibers produced from fluoroalkyl acrylates and methacrylates with a fluorinated alkyl moiety of up to 3 carbon atoms, and copolymers of these fluoroalkyl groups with both fluorinated and non-fluorinated acrylates and methacrylates.
U.S. Pat. No. 4,544,235 (Nishida et al.) describes a plastic optical fiber having a cladding comprising a transparent copolymer comprising (1) about 20 to 99% by weight of a fluoroalkylacrylate, a fluoroalkylmethacrylate, or a mixture thereof, (2) 0.05 to 10% by weight of at least one hydrophilic radical and (3) optionally, 79.95% or less of at least one one vinyl monomer, different from comonomers (1) and (2).
U.S. Pat. No. 4,968,116 (Hulme-Lowe et al.) describes an optical fiber comprising a core coated with a cladding composition comprising a fluorinated mono-acrylate, a polyfunctional cross-linking acrylate being difunctional or higher, and a photoinitiator.